Organic EL displays can be broadly classified into two types according to the method by which their organic light-emitting films are formed. The first type of organic EL displays has organic light-emitting layers that are formed by vapor deposition, a technique used when low-molecular weight organic material is employed as raw material. The other type of organic EL displays has organic light-emitting layers that are formed by the coating method, a technique often used when employing high-molecular organic material as raw material, as well as when employing low-molecular weight organic material as raw material.
One of the representative approaches used to form organic light-emitting layers by the coating technique is a method that ejects ink droplets containing organic luminescent material by means of an inkjet apparatus on pixel regions of a display substrate to form organic light-emitting layers (see, e.g., Patent Literature 1). The ink droplets contain an organic luminescent material and a solvent.
An inkjet apparatus, which includes an inkjet head having a plurality of nozzles, ejects ink droplets through the nozzles while controlling the positional relationship between the nozzles and the substrate (see, e.g., Patent Literature 2). Patent Literature 2 discloses that droplets jetted onto a substrate are configured to equally spread to form a linear pixel with a given width.
Pixel regions of a display substrate on which droplets are to be ejected are often defined by partitioning walls called a bank. This is for keeping the ejected ink droplets within specific pixel regions. Banks may define each pixel region one by one, but may define each row of pixel regions of the same color (e.g., red (R), green (G) or blue (B)) one by one (see Patent Literature 3). In some cases, banks that define rows of pixel regions are called linear banks. That is, a red, green or blue organic light-emitting layer is formed in each of the regions defined by linear banks (hereinafter may also referred to as a “linear region”).
Pixel regions to be coated with ink are 40 to 60 um in size; therefore, failure to precisely eject ink droplets results in coating position misalignment. Coating position misalignment causes color mixing between adjacent pixel regions and/or layer thickness variation. As a technique for achieving precise inkjet printing, a method is disclosed in which a test coating operation is first conducted by ejecting ink droplets through all nozzles to create a test print pattern, followed by measurement of the landing position of the ink droplets on the test print pattern and by correction of the ink ejection position using the measured landing position (see, e.g., Patent Literature 4).
The technology disclosed in Patent literature 4 will be described with reference to FIG. 15. FIG. 15 illustrates microarray fabrication apparatus 90 that includes imaging means 30, drive control means 40, stage 50, carriage (head) 60, table 70, and cartridge 80. A plurality of substrates 100 as coatings target, is placed on table 70. Substrates 100 include a test substrate. Through all nozzles of microarray fabrication apparatus 90, liquid droplets are first ejected onto a test substrate to create (draw) thereon test print pattern 300; the landing position of the liquid droplets on test print pattern 300 is determined; and the ink landing position for the next ink ejection is corrected based on the measurement. In this way a microarray is fabricated on substrate 100.
Another known technique for achieving precise inkjet printing is a method in which a pattern for correcting coating position misalignment is created on a water-repellent sheet set apart from a coating target, followed by reading of the misalignment correction pattern and by correction of the target coating position based on the reading (see Patent Literature 5).
An additional known technique for achieving precise inkjet printing involves placement of an alignment mark at a non-coating region beside a coating region of the substrate to be coated (see Patent Literatures 6 and 7). The alignment mark placed on the substrate is then captured by a camera, calculating the position for the next ink ejection based on the position of the alignment mark. Ink is then applied on the substrate.